Making Sense of the Glycemic Index

By Jeff Behar, MS, MBA

What is the Glycemic Index?

Not all carbohydrate foods are created equal, in fact they behave quite differently in our bodies. The Glycemic index (also Glycemic index, GI) is a ranking system invented by Dr. David J. Jenkins and colleagues in 1981 at the University of Toronto that ranks the effects of carbohydrates on blood glucose levels. The glycemic index calculates how high your blood sugar rises in two hours after you eat a food containing roughly 50 grams of carbohydrates, compared to how much it rises after you eat a 50 gram serving of white bread or 50 grams of pure glucose (sugar). The higher the GI for a certain food, the faster your body absorbs the carbs from that food. A lower GI means a food has a slower rate of carbohydrate absorption, and thus lower blood sugar and insulin peaks.

How the Glycemic Index Works

Carbohydrates that break down slowly, releasing glucose gradually into the blood stream, have a low glycemic index. A lower glycemic index suggests slower rates of digestion and absorption of the sugars and starches in the foods and may also indicate greater extraction from the liver and periphery of the products of carbohydrate digestion. Examples of some low GI foods include:

The Glycemic Load: A Better Measure

The GI tells you how quickly a particular carbohydrate in food makes your blood sugar rise, but it doesn't take into account how many carbohydrates are found in a serving. The glycemic load, however, takes the number of carbs per serving into consideration along with the food's glycemic index. To find a food's glycemic load, you basically multiply its GI value by number of carbohydrates per serving. Therefore, the glycemic load allows us to compare the likely effect on blood sugar of realistic serving sizes of different foods.

What Influences the Glycemic Load/Index?

Many factors help determine your body's glycemic response to a particular food, including:

• Physical form, such as a whole apple vs. applesauce. Mashing foods tends to give them a higher glycemic index/load.
• Processing. The more processed or refined a food, generally, the higher its glycemic index/load will be.
• Ripeness. The riper the fruit, the higher its glycemic index.
• Acidity. The higher a food's acidity, the lower its glycemic index/load.
• Fiber -- particularly viscous fiber, a type of soluble fiber found in oats, barley, and other foods. Generally, the higher the fiber, the lower the glycemic index/load. Whether protein and fat were eaten with the food. The presence of high amounts of protein and fat will decrease the glycemic index/load.

The following foods, even in large amounts, when eaten alone are not likely to cause a significant rise in blood sugar because they contain little carbohydrate: meat, poultry, fish, avocados, salad vegetables, eggs, fish, and cheese.

Glycemic Load and Index Values for Common Foods

Here are glycemic index (GI) and glycemic load (GL) values for some common foods. I have included their fiber content as well. Keep in mind that GI/GL is just one tool. Other aspects of food, like vitamin, mineral, fiber, and phytochemical content, are also very important.

This table uses white bread as the reference for glycemic index. White bread has a glycemic index of 100 when used as the reference food.

Glycemic Load and Index Values for Common Foods

Here are glycemic index (GI) and glycemic load (GL) values for some common foods. I have included their fiber content as well. Keep in mind that GI/GL is just one tool. Other aspects of food, like vitamin, mineral, fiber, and phytochemical content, are also very important.

This table uses white bread as the reference for glycemic index. White bread has a glycemic index of 100 when used as the reference food.

So, What's the Bottom Line?

I always look for the bottom line. And in the case of glycemic load, it tends to lead you to less-processed types of carbohydrate-rich foods, like vegetables, fruits, whole grains, and beans/legumes.

The truth is, there is plenty of evidence that a mostly plant-based diet can reduce your risk of diseases like cancer, heart disease, and diabetes. And these foods tend to have lower glycemic index numbers.

But we have yet to determine whether a low-glycemic-index diet is really what helps prevent disease, or whether this effect comes mostly from eating a healthful variety of foods.